Method for Hot Isostatic Pressing

ABSTRACT

A method for the treatment of metal articles by hot isostatic pressing, wherein air is used as the pressure medium for the hot isostatic pressing.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage of International Application No. PCT/EP2006/007257, filed on Jul. 24, 2006, which claims the benefit of European Patent Application No. EP2005/008063, filed on Jul. 25, 2005, the full contents of said prior applications being incorporated herein by reference.

TECHNICAL FIELD

The present invention concerns a method and a corresponding use for hot isostatic pressing of a metal article.

TECHNICAL BACKGROUND

Hot isostatic pressing (HIP) is a technology that finds more and more widespread use. Hot isostatic pressing is for instance used in achieving elimination of porosity in metal castings, such as for instance turbine blades. This will substantially increase the service life and strength of the castings, in particular the fatigue strength. Another field of application is the manufacture of products with fully dense and pore-free structures by compressing powder.

Examples of devices and methods for hot isostatic pressing are disclosed in WO 00/15371, WO 01/14087, and WO 03/070402, which are all incorporated herein by reference.

In hot isostatic pressing of metal articles, the metal article to be subjected to treatment by pressing is positioned in a load compartment of a pressure vessel. The vessel is sealed off and a pressure medium is introduced into the pressure vessel and the load compartment thereof. The pressure medium is heated and the pressure is increased such that the article is simultaneously subjected to an increased temperature and an increased pressure during a selected period of time. The pressures, temperatures and treatment times are of course dependent of many factors, such as the material of the pressed article, the field of application, etc. The pressures can range from 200-5000 bar, and the temperatures from 300-3000° C.

A drawback of the existing hot isostatic pressing technology for treating metal articles is the high costs for providing the pressure medium, which traditionally is argon or nitrogen.

SUMMARY OF THE INVENTION

One object of the present invention is to provide hot isostatic pressing in which the costs involved for providing pressure medium is reduced.

This object is achieved by providing a method for hot isostatic pressing and corresponding use as claimed in the independent claims. Embodiments are defined in the dependent claims.

According to a first aspect of the invention, there is provided a method for hot isostatic pressing of articles in a press comprising a heat-insulated pressure vessel, a furnace chamber enclosed in the pressure vessel, and a load compartment arranged inside the furnace chamber. The method comprises the steps of positioning an article to be pressed inside the load compartment, said article being made of metal, feeding pressure medium into the pressure vessel, said pressure medium being air, increasing the temperature and the pressure in the load compartment, maintaining the increased pressure and the increased temperature for a selected period of time, and reducing the temperature and the pressure in the load compartment.

According to a second aspect of the invention, there is provided a method of hot isostatic pressing of an article made of metal. The method comprises the steps of providing air as a pressure medium, and using air as said pressure medium for the hot isostatic pressing.

Thus, the invention is based on the advantageous idea of using air as the pressure medium during hot isostatic pressing of a metal article in a high-pressure press. Needless to say, this goes against the well-established prejudice within the art of hot isostatic pressing of metal articles. As is well known to persons skilled in the art, the presence of air is generally detrimental to the hot isostatic pressing process of metal articles, since the metal may interact with the oxygen or nitrogen content of air, or both, for instance forming oxides or nitrides, or reducing the kindling temperature of the metal such that ignition of the metal may occur at the pressures and temperatures used for hot isostatic pressing.

Another reason for the strong prejudice within the art against using air in any type of hot isostatic pressing, is the fact that the material of the pressure chamber portions coming into contact with the pressure medium must also be able to endure or withstand the detrimental effects of oxygen and/or nitrogen at hot isostatic pressing temperatures and pressures. For instance, material transport within the pressure chamber may occur, and oxygen and nitrogen may be large contributors to chemical erosion at the high hot isostatic pressing temperatures and pressures.

It may, however, be noted that the use of air in hot isostatic pressing is known per se, for instance from the Japanese patent publication JP 2-129077 which discloses a method for producing sintered ceramics of high density. However, the use of air disclosed in JP 2-129077 as a pressure medium for hot isostatic pressing is only related to the treatment of ceramics, where the use an inert gas such as argon may lead to undesired removal of oxygen from the treated ceramics. Thus, the use of air in the disclosed method is intended for preventing said removal of oxygen from the ceramics.

Another method for a similar purpose is disclosed in Japanese patent publication JP 61-23732, in which oxygen, O₂, is added to an inert gas for avoiding undesired removal of oxygen from treated ceramics. However, it should be noted that JP 61-23732 is silent about using air in the HIP process. Instead, air is exhausted from the pressure chamber before feeding argon and oxygen into same.

It should be noted that for hot isostatic pressing of ceramics in an air or oxygen containing atmosphere, as disclosed in the above mentioned Japanese publication, the materials that must be used for composing the interior of the press are different to materials used in an inert gas environment. The reason is mainly that the material must be resistant to the detrimental effects of the oxygen at the high pressures and temperatures at HIP processing of ceramics. This applies to the heating elements, the article holding arrangements, the linings, the internal housing structure of the press, etc. Often, the internally located components of the press must be made of ceramics, which significantly increases the overall manufacturing and maintenance costs for the press. Also, a press where the internal body is made of ceramics is both difficult and expensive to scale of to full size hot isostatic pressing. To the skilled person, this is yet another deterrent from using air as a pressure medium in HIP processing, which strengthens the prejudice against using air as pressure medium in HIP processing of metal articles even further.

Of course, the heat treatment of metal articles in air is nothing new. However, the use of air as a pressure medium in hot isostatic pressing is an entirely different matter, as compared to heat treatment at atmospheric pressure. The effects of a specific gas on a specific material is generally very much dependent on the pressure. Thus, at temperatures where a gas would not have a negative effect on a particular material at atmospheric pressure, the very same gas may be positively devastating for the same material at the same temperature in a high pressure environment, such as at HIP pressures.

The advantages of using air as the pressure medium for hot isostatic pressing of metal articles are manifold. For one, ambient air may be used as the pressure medium. Thereby, there is no need for transporting the pressure medium to the processing site. Furthermore, when a typical gas is to be used as pressure medium, it is generally transported and stored in liquid state. Thus, there exists a need for cryogenic storage facilities and tanker trucks, as well as cryogenic pumps used in connection therewith, which are all very expensive. Also, the costs for extracting and isolating traditional pressure mediums, such as nitrogen or argon, are also substantial, in particular for argon. Accordingly, the basic costs for acquiring a typical pressure medium, even when disregarding the costs involved for transportation and storage, remain a major factor in the overall processing costs.

A further advantage of the invention is that the need for evacuating the pressure vessel from air before introducing the pressure medium can be completely omitted, which reduces processing time and, hence, costs. In other words, the processing costs for performing the hot isostatic pressing can be dramatically reduced when using air as the pressure medium.

Also, the use of asphyxiating or choking gases, such as nitrogen and argon, as pressure medium entails cumbersome and sometimes expensive safety measures to be adopted. For one, the procedures connected with the actual handling of asphyxiating gases must ensure that the staff does not come into unintentional contact with the asphyxiating gases, or that the gases are prevented from mixing with the ambient air. If an operator for reasons of maintenance or inspection would be required to enter into the pressure chamber or the pit where the press is situated, it must be ensured that all asphyxiating pressure medium has been evacuated. Gauges for measuring the degree of asphyxiating gases within the press and in its surroundings must be provided, as well as special training for the staff handling the presses. By using air as the pressure medium for hot isostatic pressing of metal articles, such measures can be essentially omitted.

It should be noted that not all metals are suitable to be subjected to hot isostatic pressing using air as the pressure medium, which is mainly due to detrimental effects of the oxygen and/or nitrogen content in air. Nevertheless, it has surprisingly been found, against the prejudice within the art, that some metals have properties that render hot isostatic pressing by air possible.

As an example thereof, aluminium and alloys thereof have been found to be well suited for hot isostatic pressing by air. Although the aluminium reacts very rapidly with the oxygen content in air, and an aluminium oxide layer is rapidly formed on the surface of an aluminium article, it is still possible to successfully perform hot isostatic pressing of aluminium or aluminium alloys using air as the pressure medium. The reason is that the denseness of the oxide layer formed on the aluminium surface prevents air from coming into contact with the bare metal and, thereby protects the material during the hot isostatic pressing. Furthermore, when subjecting the aluminium or the aluminium alloy to air during the hot isostatic pressing, the density of the oxide layer will increase even further.

Furthermore, other metals such as magnesium and alloys thereof may be suitable for hot isostatic pressing by air. Thus, it should be noted that the present application is not restricted to hot isostatic pressing using air as pressure medium for aluminium and aluminium alloys alone, but other metals are also contemplated within the scope of this application.

As stated above, and according to the invention, air is used as said pressure medium. In other words, the air present in the surroundings of the press, or in the near vicinity thereof, i.e. ambient air, can be used as the pressure medium. In some embodiments, a gas or air storage could be used, in which air is stored under pressure in order to facilitate and speed up the process of feeding air into and increasing the pressure in the pressure vessel to the desired treatment pressure. Then, for some embodiments, a compressor could be used for feeding the air into the storage and/or for feeding the air from the storage to the pressure vessel.

When an air storage is used, the air present in the pressure vessel during the hot isostatic pressing process could be fed back to the air storage following completion of the process and re-used in subsequent pressing cycles. This would reduce the performance requirements for a feeding device, such as a compressor, provided between the air storage and the pressure vessel.

Alternatively, the air used in the pressure chamber could simply be released into the atmosphere between subsequent pressing cycles. Then, the release of the pressure medium could be done without any detrimental effects on the environment, or any risk of asphyxiation for staff or other people present near the press or the pressure medium outlet thereof.

Furthermore, air could be supplied in liquid state to the site or plant where the hot isostatic pressing is performed, such as by cryogenic tanker trucks, cryogenic pipelines, cryogenic pumps, etc., and stored in cryogenic storage facilities.

In further exemplifying embodiments, a dehumidifier is used for reducing the humidity of the pressure medium, i.e. the air. This is due to the fact that ambient air contains a certain amount of water, in vapor or liquid form. By reducing the humidity, detrimental effects that could arise from the presence of water within the pressure vessel, is greatly reduced. Thereby, the reliability and service life of components within the pressure vessel, such as electrical feed-through, sealing surfaces, electrical terminals, insulation materials, etc., can be increased.

Also, as stated above, suitable pressures and temperatures for the hot isostatic pressing process is dependent on the particular material and the field of use for the metal article to be pressed. Nevertheless, in exemplifying embodiments, the pressure can be in the range from 200 to 3000 bar, with the temperature ranging from 300 to 1000° C.

For embodiments related to hot isostatic pressing of aluminium or aluminium alloys, the typical pressure and temperature ranges can be 300-600 bar and 400-600° C., respectively. At such relatively low HIP temperatures and pressures, the detrimental effects of the oxygen and nitrogen contents of air are different to that of higher temperatures and/or pressures, for instance as used in hot isostatic pressing of ceramics. Therefore, even though the material costs involved for the internal components of a press using air for hot isostatic pressing of aluminium or aluminium alloys would be greater than for a corresponding press using e.g. argon or nitrogen, it is still significantly less than for a corresponding press for hot isostatic pressing of ceramics.

When using air as the pressure medium, the materials used for the parts and components of the pressure vessel, particularly those arranged in the interior of the pressure vessel, are suitably selected in adaptation to the air environment. For instance, the material of the furnace, or of heating elements thereof, could for example be chosen from materials that do not react or are negatively effected by air at the temperatures and pressures that the furnace is subjected to during the hot isostatic pressing process. Thus, the scope of the present invention is by no means restricted to particular materials. Any material suitable for the furnace when using air as the pressure medium during hot isostatic pressing is contemplated within the scope of the present invention.

Further objects and advantages of the present invention will be discussed below by means of exemplifying embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplifying embodiments of the invention will be described below with reference to the accompanying drawings, in which:

FIG. 1 is a schematical illustration of a hot isostatic pressing arrangement suitable for use in embodiments of the present invention;

FIG. 2 is a schematic illustration of a further hot isostatic pressing arrangement suitable for use in embodiments of the present invention; and

FIG. 3 is a schematic illustration of yet another hot isostatic pressing arrangement suitable for use in embodiments of the present invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

The following is a description of exemplifying embodiments in accordance with the present invention. This description is intended for further elucidating the general principles of the invention and is not to be taken in a limiting sense. It must also be noted that the drawings are schematic and that the pressing arrangements of the described embodiments comprise a number of features and elements that, for ease of description, are not indicated in the drawings.

With reference first to FIGS. 1 and 2, there is shown a high-pressure press for hot isostatic pressing of metallic articles. The press includes a pressure vessel 1 having a thermally insulated casing 2 which thermally seals off the interior of the pressure vessel 1 and reduces heat loss. Even though this is not shown in the figure, the pressure vessel 1 may be opened, such that the contents of the pressure vessel 1 can be removed. Furthermore, the wall of the pressure vessel 1 can be provided with channels (not shown) for water cooling of the vessel wall in order to protect it from detrimental heat.

The interior of the casing 2 forms a furnace chamber 3, which during operation of the press is sealed off from the surroundings of the pressure vessel. A portion of the furnace chamber 3 is intended for receiving and holding the metal articles 7 to be pressed, which portion is in the following referred to as a load compartment 4. In the load compartment, there is generally arranged a holding arrangement 8 for holding or supporting the metal articles 7 to be pressed. Even though denoted as a load compartment, depending on the configuration of the press and of the metal articles to be pressed, the load compartment 4 can also be divided into a number of sub-compartments for holding metal articles 7. One such example is shown in FIGS. 1 and 2, in which the load compartment 4 is defined by the holding arrangement 8 and divided into four separate portions, separated by grids or perforated shelves 9 of the holding arrangement 8 that allows for hot pressure medium to flow around the metal articles 7 held by the shelves 9. Of course, solid shelves 9 may also be used for holding the metal articles 7.

The furnace chamber 3 further comprises a furnace 5 having heating elements for increasing the temperature of the furnace chamber 3 and, hence, the load compartment 4. In the schematic illustrations of FIGS. 1 and 2, the furnace 5 is arranged at the sides of the furnace chamber 3, i.e. adjacent and surrounding the load compartment 4. Even though this may be suitable for some embodiments, other configurations are also contemplated within the scope of the invention. For instance, heating elements of the furnace could also or alternatively be provided at the bottom of the furnace chamber 3, i.e. below the load compartment.

The heating elements of the furnace 5 are suitably made from a material that is not negatively affected by air, and in particular the oxygenous environment which is formed within the pressure vessel when using air as the pressure medium, at the particular pressures and temperatures that the furnace material is subjected to. As mentioned above, said pressures and temperatures can differ substantially between different pressing processes dependent on the application and material of the metal articles being pressed. Thus, as understood by the person skilled in the art, a number of different materials may be used, as long as the materials used are resistant to the detrimental effects of air, in particular the oxygen thereof, in a hot isostatic pressing process. For instance, use can be made of oxygen resistant heating elements currently available on the market, e.g. ceramic heating elements well known in the art.

In order for pressure medium to be provided into the pressure vessel 1 after the vessel has been sealed off, a conduit 6 is arranged for delivering the pressure medium to the pressure vessel 1, and for releasing the pressure medium from the vessel 1 when the pressing cycle has been completed. The conduit is provided with a valve (not shown) for opening the conduit 6 during the delivery and release of pressure medium, and closing the conduit 6 during the pressing process. Of course, the pressure vessel could be provided with a plurality of conduits for delivering and releasing pressure medium. For example, there could be one or more conduits for the release of pressure medium that are separate from conduit(s) for the delivery of pressure medium. Furthermore, a plurality of separately controllable conduits could be used for accurately controlling and varying the flow rate of pressure medium to and from the pressure vessel, in particular for variably controlling the release of pressure medium from the pressure vessel.

Furthermore, the arrangement shown in FIG. 1 comprises a compressor 10 for feeding air as the pressure medium into the pressure vessel 1. The compressor 10 comprises an outlet 11 connected to the conduit 6, and an inlet 12 for providing air to the compressor. In the embodiment shown in FIG. 1, the compressor inlet 12 is arranged for receiving ambient air from the surroundings of the pressing arrangement. Thus, no further storage facilities are provided for the pressure medium.

In FIG. 2, the pressing arrangement further comprises a pressure medium storage 20, which for the present embodiments is in the form of an air tank since the pressure medium is air. According to these embodiments, air is stored under pressure in said tank 20, such that the time for providing pressure medium to the pressure vessel and increasing the pressure thereof to the intended pressure level can be reduced. The air tank preferably comprises a compressor 21 arranged for feeding air into the tank 20 and increasing the air pressure in the tank. As readily understood by a person skilled in the art, use can be made of a single compressor for feeding air into the storage tank 20, as well as for feeding pressure medium into the pressure vessel, thus replacing the two compressors 10, 21 illustrated in FIG. 2. Then, means are suitably provided for controlling the feeding direction of such a single compressor.

Furthermore, when using an air storage tank 20 for supplying air to the pressure vessel, the need for a compressor 10 between the tank 20 and the pressure vessel 1 may be reduced. Thus, the feeding device 10 shown in FIG. 2 could be a compressor, but could also be another type of feeding or control device for delivering air to a desired pressure level from the storage 20 into the pressure vessel 1. In other words, if the air pressure of the storage tank 20 exceeds the air pressure which is to be delivered to the pressure vessel via the conduit 6, the storage tank 20 could then be essentially directly connected to the conduit 6. Thus, an outlet of the storage tank 20 could be essentially directly connected to the conduit 6 of the pressure vessel 1.

Furthermore, the air supplied to the storage tank could be ambient air, taken directly from the surroundings of the pressing arrangement or of the manufacturing plant at which the press is located. Alternatively, air could be provided from an external source, such as via tanker trucks or a pipeline. In particular when it is desired to store air in liquid form, as referred to below.

Thus, in some embodiments of the invention, the pressure medium storage 20 may be in the form of a cryogenic storage tank. Then, collected air has been cooled to a temperature and at a pressure at which a transition from gas to liquid state has been achieved. Accordingly, the air is stored in liquid form in the storage tank 20. When a cryogenic tank is used as said pressure medium storage 20, a cryogenic pump is then preferably used as said feeding device 10 for delivering the air to the pressure vessel 1.

Turning now to FIG. 3, there is shown an illustration of further exemplifying embodiments of the present invention. According to these embodiments, a dehumidifier 30 is provided for dehumidifying the pressure medium, i.e. to remove or at least reduce the water contents of the air used as the pressure medium.

In the shown example, an inlet 32 of the dehumidifier 30 is connected to the outlet of the compressor 10, and an outlet 32 of the dehumidifier 30 is connected to the pressure vessel conduit 6. The illustrated example further comprises a storage tank 20 connected to the compressor 10. However, the use of a dehumidifier is not restricted to the presence of a storage tank.

Furthermore, and in analogy with the embodiment referred to above in which the storage tank 20 is directly connected to the pressure vessel 1, a dehumidifier 30 could be arranged between the compressor 21 and the storage tank 20 (not shown). Thus, the water content of the air to be stored is reduced before the air is fed into the storage tank 20, which reduces the detrimental effects that could arise from the presence of water within the air tank.

Moreover, for the embodiments in which two feeding devices or compressors 10, 21 are used, i.e. in analogy with the arrangement described in relation to FIG. 2, two dehumidifiers can be used (not shown). Then, one dehumidifier may be provided after the compressor 21 for feeding air into the storage tank 20, and one after the compressor 10 for feeding stored air into the pressure vessel 1, as seen in the air feeding direction.

However, it should be noted that described embodiments comprising a dehumidifier are merely exemplary and not to be taken in a limiting sense. Thus, the scope of the present invention is by no means restricted to dehumidifying the air before use thereof as the pressure medium.

A pressing operation in accordance with exemplifying embodiments of the present invention will now be described. First, the pressure vessel 1 is opened such that the furnace chamber 3, and the load compartment 4 thereof may be accessed. This can be accomplished in a number of different manners known in the art and no further description thereof is required for understanding the principles of the invention.

Then, the metal articles 7 to be pressed are positioned in the holding arrangement 8 of the load compartment 4 and the pressure vessel 1 is closed.

In the embodiments now described, using the temperatures and pressures mentioned below, the metal articles are made of aluminium or aluminium alloys. However, and as mentioned above, other materials suitable for hot isostatic pressing by air are also contemplated within the scope of the present invention, such as magnesium and alloys thereof.

When the metal articles 7 have been positioned in the load compartment 4 of the pressure vessel 1, pressure medium in the form of air is fed into the pressure vessel 1 via the conduit 6, for instance using a compressor 10, a pressurized storage tank 20, a cryogenic pump, or the like. The feeding of air into the pressure vessel 1 continues until a desired air pressure is obtained inside the pressure vessel 1.

While feeding air into the pressure vessel 1, the heating elements of the furnace are activated and the temperature inside the load compartment 4 is increased. Suitably, the feeding of air continues and the pressure is increased until a pressure level has been obtained that is below the desired pressure for the pressing process, and at a temperature below the desired pressing temperature. Then, the conduit 6 is sealed and the temperature in the furnace chamber 3 is increased using the heating elements of the furnace, such that the pressure rises to reach the desired pressing pressure.

In accordance with the embodiments presently described, that is using air as the pressure medium for hot isostatic pressing of aluminium or aluminium alloys, the desired pressure is in the range of 300-600 bar, and the desired temperature is in the range 400-600° C.

Following a selected time period at which the temperature and pressure is maintained, i.e. the actual pressing period, the pressure of the pressure vessel is released by opening the conduit 6. The air used during the pressure cycle can then simply be discharged into the surroundings of the pressure vessel, or, in particular when a storage tank for storing air in gas form is used, can be fed back into the storage tank 20 using a suitable feeding device, such as a compressor 10. If the air used during the pressing cycle is recycled into an air storage, the performance or capacity requirements of the air feeding device can be reduced, and a smaller compressor may consequently be used.

Following the completed hot isostatic pressing cycle, the metal articles 7 are removed from the press 1 and cooled. According to the described embodiments, the pressure vessel 1 is opened such that the entire holding arrangement 8 can be separated from the pressure vessel 1. Thus, the holding arrangement 8 may also hold the metal articles thus subjected to the pressing treatment during the cooling phase of the hot isostatic pressing cycle.

Even though the present description and drawings disclose embodiments and examples, including selections of components, materials, temperature ranges, pressure ranges, etc., the invention is not restricted to these specific examples. Numerous modifications and variations can be made without departing from the scope of the present invention as defined in the accompanied claims. 

1.-11. (canceled)
 12. A method for hot isostatic pressing of articles in a press, the press including a heat-insulated pressure vessel, a furnace chamber enclosed in the pressure vessel, and a load compartment arranged inside the furnace chamber, the method comprising: positioning articles to be pressed inside the load compartment, said articles being made of metal; feeding pressure medium into the pressure vessel, said pressure medium being air; increasing the temperature and the pressure in the load compartment; maintaining the increased pressure and the increased temperature for a selected period of time; and reducing the temperature and the pressure in the load compartment.
 13. The method as claimed in claim 12, wherein said air is ambient air.
 14. The method as claimed in claim 12, further comprising: storing pressurized ambient air in a gas storage; and feeding said pressurized air from the gas storage into the pressure vessel as said pressure medium.
 15. The method as claimed in claim 12, further comprising: storing air in liquid form in a cryogenic storage; and feeding said stored air from the cryogenic storage into the pressure vessel as said pressure medium.
 16. The method as claimed in claim 12, further comprising: dehumidifying the air to be used as said pressure medium.
 17. The method as claimed in claim 12, wherein, in increasing the pressure in the load compartment, the pressure is increased to 200-1000 bar.
 18. The method as claimed in claim 17, wherein, in increasing the pressure in the load compartment, the pressure is increased to 300-600 bar.
 19. The method as claimed in claim 12, wherein the temperature, in increasing the temperature in the load compartment, is increased to 400-600° C.
 20. The method as claimed in claim 12, wherein said metal is selected from the group of aluminium and aluminium alloys.
 21. A method of hot isostatic pressing of an article made of a metal, comprising: providing air as a pressure medium; and using air as said pressure medium for the hot isostatic pressing.
 22. The method as claimed in claim 21, wherein said air is ambient air.
 23. The method as claimed in claim 21, wherein said metal is selected from the group of aluminium and aluminium alloys.
 24. The method as claimed in claim 13, further comprising the steps of: storing pressurized ambient air in a gas storage; and feeding said pressurized air from the gas storage into the pressure vessel as said pressure medium.
 25. The method as claimed in claim 13, further comprising: storing air in liquid form in a cryogenic storage; and feeding said stored air from the cryogenic storage into the pressure vessel as said pressure medium.
 26. The method as claimed in claim 13, further comprising: dehumidifying the air to be used as said pressure medium.
 27. The method as claimed in claim 13, wherein, in increasing the pressure in the load compartment, the pressure is increased to 200-1000 bar.
 28. The method as claimed in claim 27, wherein, in increasing the pressure in the load compartment, the pressure is increased to 300-600 bar.
 29. The method as claimed in claim 13, wherein the temperature, in increasing the temperature in the load compartment, is increased to 400-600° C.
 30. The method as claimed in claim 13, wherein said metal is selected from the group of aluminium and aluminium alloys.
 31. Method as claimed in claim 22, wherein said metal is selected from the group of aluminium and aluminium alloys. 